Posted
by
kdawson
on Tuesday January 19, 2010 @03:58PM
from the it's-the-law dept.

thomst passes along news out of the recent AAAS meeting of a new explanation for pulsar beams that involves faster-than-light currents. Here are Los Alamos's press release and threerelatedpapers on the arXiv. "The new model explains the beam emissions from pulsars as products of superluminal currents within the spinning neutron stars' atmospheres. According to the authors' model, the current generated is, itself, faster than light, although the particles that compose it never individually exceed the universal speed limit, thereby preventing Einsteinian post-mortem rotation. The new model is a general explanation of the phenomenon of pulsar beam emissions that explains emissions at all observed frequencies (and different pulsars emit everything from radio waves to x-rays), which no previous model has done."

Can we replicate this and add information to the current to transport information faster than the speed of light? (The real problem.)

Well I'm going to say no simply based on the fact that they are claiming no physical laws are being broken and that Special Relativity is not violated, since super-luminal information transfer = time travel = causality violation = impossible in SR. This not the first time this effect has been proposed and it has apparently been studied in labs, so if it was a possible way to transmit information, it seems they would have probably figured that out by now and at least some aspect of SR (perhaps causality!) would have to be scrapped.

I don't fully understand what they're talking about, but it sounds like a similar phenomenon to group velocity [wikipedia.org], in which some aspect of the wavefront can be said to be traveling faster than light, but nevertheless real photons and information cannot.

We know the speed of light in some mediums is less than c (see cherekov radiation). Is it possible the speed of light is greater than c in some mediums? You have to admit that a neutron star is pretty exotic stuff. What about negative-index metamaterials? Beyond that (and this may be non sequitur) maybe a concentration of "dark energy" has properties we don't understand.

It's interesting to watch as the/. crowd's replies to the technical question in the post become both less succinct and more smarmy as you get further down the comments. I have not yet attempted to correlate this phenomenon to user id number.

Yeah, it *is* the same stuff as always, it's only the distance that changed. It was theorized that the particles could *not* remain entangled for nearly that distance -- that makes all the difference.

Ok, I know the following will sound like a "loophole", but we need to define "transfer of information" here. If I send a batch of particles a light-year away in a certain direction, and then "store" them (prevent them from interacting), then I've created the potential for faster-than-light communication. The particles, once they reach their destination, will be under constant observation. When I want to send a signal, I make the particles on my end "react" in a pulse-width-modulation fashion, like Morse code. Their corresponding particle pairs on the other end will "untangle" at the same rate, but instantaneously.

If I have this wrong please correct me. This at least was my understanding of the process. It's my understanding that the distance used to be limited, which is why it wasn't realistic, but it appears that the distance is irrelevant (either that, or we haven't managed to detect any relevance yet).

Your post is basically on the right track, but some thing you say are not quite right.

There are many concepts in our current understanding of physics that you just take to be inviolate like conservation of energy, momentum, speed of light.

Well, not quite.

In flat spacetime, velocities greater than c lead to violations of causality: observer 1 says that event A caused event B, but observer 2, in a different state of motion, says that B caused A. Since violation of causality can produce paradoxes, we suspect that cause and effect can't be propagated at velocities greater than c in flat spacetime.

In curved spacetime, this is far from being established. General relativity has spacetimes, such as the Godel solution, that are valid solutions of the field equations, and that violate causality. Hawking's chronology protection conjecture says that this kind of causality violation can't arise from realistic conditions in our universe -- but that's all it is, a conjecture. Nobody has proved it. In fact, there is a major current research program that consists of nothing more than trying to *define* rigorously what the chronology protection conjecture means.

Physics develops from what proceeded it, from Newton to Einstein to Quantum Mechanics to String Theory, and those conservation laws always held.

Okay, but the prohibition on transmission of cause and effect at velocities greater than c isn't a conservation law.

You don't need to know the details of how a proposed "perpetual motion machine" may work to know that if the crackpot building it says that it violates the law of conservation of energy then it doesn't work.

I think the analogy here would be the following. Even the slashdot summary makes it clear that they aren't really claiming propagation of information at velocities greater than c. That's also reasonable, because although a neutron star is a relativistic object, it's not all that highly relativistic. Its structure is complicated from a nuclear physics point of view, but from the point of view of the relativistic description, it's a very plain vanilla solution of the Einstein field equations. If information was going to be transmitted at greater than c, then the chronology protection conjecture would also be violated, but that's not going to happen in such an ordinary, well studied spacetime.

It is not safe to use your criterion to rule out examples from general relativity without more attention to the details. Based on your criterion, the Godel spacetime has to be a crackpot idea, and so is the Alcubierre drive. In reality, there is a clear consensus among relativists that the spacetimes found by Godel and Alcubierre are correct -- it's just not clear how to interpret them, or whether they could actually arise from realistic conditions in our universe.

It's not so much needed a better system of propulsion. It's that, as you approach light speed, your mass increases. This means you need more fuel to push yourself faster. This more fuel increases your mass, which is still increasing exponentially as you get closer and closer to light speed.

The exact formula is:

M = MassAtRest / sqrt(1 - (v/c)^2)

At 0.5c, your mass would be about 1.3 times your rest mass. At 0.9c, you'd be nearing 2.3 times rest mass. At 0.99c, you'd have passed 7 times rest mass. At 0.999c, 22 times rest mass. And so on.

Now what happens if you go faster than light? (Supposing you somehow "skip over" the light speed barrier.) You get into imaginary numbers. For example, at 2c your mass would be MassAtRest / sqrt(-3). What does that imaginary number translate into? There are many theories, but no firm answers. The equations for velocity and time are similar so some theorize that it means going back in time. Others say the imaginary numbers mean it just can't be done. Still others think that this just shows where relativity breaks down and a new set of equations is called for.

The spots won't be moving faster than light, they will actually be a blur or line spread across the surfaces they hit.

You're confusing perception with reality, and they are two very different things.

I think the GP was right, any you may have it backwards. The human eye will perceive a blur or line, due to the limited "frame refresh" and averaging of our optical system. In reality, though, the "spot", as defined by the location where the photons are hitting/reflecting from the surface, will be traveling faster than light. No information can be conveyed, however, as no point on this surface can directly use this phenomenon to actually communicate anything faster than light.

This is where you need special relativity. Nothing can move faster than the speed of light relative to anything else, regardless of what reference frame the observer is in. If each train is traveling 1/10c relative to the train below it, an observer on the ground will see the twelfth train traveling at 0.869c.

Also, due to length contraction, if the first train was long enough to reach around the globe when stationary, it will be some 200 km too short when traveling at 0.1c, each subsequent train will be shorter yet.

My understanding from the intro subjects I took while avoiding doing my advanced engineering electives is that while the speed of light in a given medium can change, the speed of light in a vacuum is the c we're talking about when we say you can't go faster than it.

From wikipedia:
"It is important to note that, at a microscopic level, the speed at which the photons travel is always the same. That is, the speed of light, commonly designated as c, does not change. The light appears to travel more slowly while traversing a medium due to the frequent interactions of the photons with matter. This is similar to a train that, while moving, travels at a constant velocity. If such a train were to travel on a set of tracks with many stops it would appear to be moving more slowly overall; i.e., have a lower average velocity, despite having a constant higher velocity while moving."

So the speed of light isn't actually changing in the medium; only the aggregate observed speed at a macroscopic level. Microscopically, light always travels at the same constant c, and the refractive index is reflective of the property of the material which tends to make the light take a longer path. The longer path results in the group velocity being slower than the individual velocity.

Wikipedia's information on negative-index lightspeed is much more opaque (pardon the pun):
"However, group velocity can become negative, and exceed the velocity of light c, in the particular case of anomalous dispersion. As a result, a burst of a laser's pulse will appear to exit the rear side of the negative index metamaterial before the laser pulse appears to enter the material. However the speed of transmitting information is always limited to c."

This result, as described, seems as though you ARE transmitting information faster than c, and I suspect the author of the wikipedia text realized this and just added the assertion that it doesn't, rather than re-working the example so it's correct. Alternatively, the 'observation' of the beam leaving the material before it entered it is an artifact of the observation process.

I would suggest that, based on the mechanism for the apparent slowing of light in a standard material, nothing about a negative-index material should be able to actually SPEED photons, as nothing in a normal material actually SLOWS photons, but I feel like I'm using wikipedia to falsify itself, and you could argue about the reliability of my source.

To sum up:
There are three 'velocities' of interest wrt light: phase velocity, group velocity, and photon velocity. Any of the former two can, in exotic setups, exceed c (and I do NOT grok the consequences); the latter is always precisely c.
The speed of propogation of information seems to be limited to the lesser of: 1. The photon velocity, and 2. The greater of the phase and group velocity.

Don't ask me to back this up; I've already exceeded the limit of MY speed of propagation of information (from wikipedia to brain).